Pigments having distinct hue and high coloring power are practically important, and are in the form of fine particles whose particle size is greatly reduced. Reduction of the particle size brings about the high coloring power. However, dispersion of the pigments having the reduced particle size is apt to show a high viscosity. In the case that the dispersion having high viscosity is prepared in an industrial scale, problems of difficultly taking off it from a dispersing machine and scarcely transporting it through a pipe line occur. Further, the dispersion occasionally comes unemployable due to gelation produced during its storage.
A photosensitive colored resin composition containing the above pigments of reduced particle size is useful as image forming material for preparing a color proof or a color filter which needs distinct hue and high coloring power.
A colored image is, for example, prepared using the photosensitive colored resin composition by the steps of: forming a layer of the photosensitive colored resin composition on a support, imagewise exposing the layer to light and developing the exposed layer. In the developing procedure, an aqueous alkaline solution is generally employed rather than an organic solvent as a developing solution in view of environmental pollution, and therefore the photosensitive colored resin layer is preferably soluble in an aqueous alkaline solution.
Further, the photosensitive colored resin composition is usually employed by dissolving it in an organic solvent to prepare a coating solution so as to reduce a time period for drying the coated layer. Thus, a binder (polymer) employable for the photosensitive colored resin composition is required to have an acidic group and be soluble in an appropriate organic solvent. The photosensitive colored resin composition is usually composed of an organic solvent, the polymer having acidic group and the pigment (especially organic pigment) dispersed therein.
The photosensitive colored resin layer (layer of photosensitive colored resin composition) is usually so formed as to have an extremely reduced thickness, which is required to show a high color strength. Therefore, it is necessary that the organic pigment is dispersed in the polymer having acidic group and the organic solvent under condition that the particle size of organic pigment is sufficiently reduced.
The photosensitive colored resin composition (containing organic pigment having highly reduced particle size) is, for example, useful for preparing a color proof or a color filter.
The color filter is usually prepared by forming red, green and blue pixels on a transparent support. The pixels are formed by various known methods such as a dyeing method, a printing method, an electro-deposition method and a pigment dispersion method. The pigment dispersion method is preferred from the viewpoints of quality of the resultant color filter and production efficiency.
The preparation of color filter according to the pigment dispersion method, is generally performed by the steps of: coating a solution of the photosensitive colored resin composition on a transparent support to form a photosensitive colored resin layer, imagewise exposing the photosensitive colored resin layer to light and developing the layer to form a pixel of a first color (e.g., red); and repeating these procedures two or more times to form pixels having colors different from the first color, whereby a pixel pattern having two or more colors is obtained.
When the particle size of the pigment employed for the photosensitive colored resin composition is not satisfactorily reduced, the transmittance of the resultant color filter lowers due to light adsorption and light scattering of the pigment. A liquid crystal display device where the color filter is arranged occasionally shows lowering of display contrast because a polarizing axis of the device is shifted due to the light adsorption and scattering (The 7th Color Optical Conference, Color Filter for 512 Color Displayed 10.4 inch TFT-LCD, Ueki, Ozeki, Fukunaga and Yamanaka, 1990). Hence, it is required to greatly reduce the particle size of the pigment.
To obtain the pigment dispersion having excellent flowability and a greatly reduced particle size, it is known to use various dispersants. The dispersants are classified into polymer dispersants and low molecular weight dispersants.
Examples of the polymer type dispersants include salt of polyacrylic acid, sodium maleate/olefin copolymer, polyester having a carboxy group at terminal (Japanese Patent Publication No. 2(1990)-34009) and polyester having acidic and basic groups obtained by using tetrakis(2-hydroxyalkyl)ethylenediamine as a starting material (Japanese Patent Provisional Publication No. 2(1990)-24523). Examples of the low molecular weight type dispersants include sorbitan fatty acid esters, polyoxyethylene alkylamine and alkyldiamine and alkanolamine derivatives (U.S. Pat. No. 3,536,510).
In the preparation of photosensitive colored resin composition comprising an organic pigment dispersed in the polymer having acidic group which is useful for preparing a color proof or color filter, it is not known as to which kind of dispersant to be used though a fluorine containing surface active agent is usually employed as an additive.